(1) Field of the Invention
The present invention relates in general to key telephone systems, and more particularly to protection of electronic key telephone system station instruments.
(2) Description of the Prior Art
Key telephone systems are well known in the art. When placing or receiving a call in such systems, the subscriber station instrument is normally connected to the central office or telephone exchange by means of two metallic conductors (transmission leads) arranged to form a subscribers loop. Such a loop includes battery power through a conventional battery feed relay or similar electronic device to energize the station instrument as well as the ability to carry audio signals in both directions. Protection against shorts in the subscriber loop is provided in such a system by the battery feed relay or electronic device.
Methods similar to those employed in the central office are used to supply power to key telephone stations when interstation communication, sometimes known as intercom service, is provided in a key system. Such a system is described in U.S. Pat. No. 4,010,332 issued Mar. 1, 1977, to Coy Edwin Matheny. This patent describes a method of supplying power to a key telephone system station over two transmission leads from a power source through line balancing apparatus, and a loop protecting series regulator. This is a generally satisfactory method of protection when the protected station has a very low power requirement.
Recently microprocessors have been incorporated into key telephone stations to implement new features. These microprocessor equipped key telephone stations require substantially greater amounts of power then previous types of key system station instruments, thus making protection by the above means impractical due to the large and expensive heat sinking which would be required for the pass elements used.
Another method of providing protection against over-current conditions while requiring less heat sinking of a pass element such as a power transistor is through the use of foldback current limiting as taught in U.S. Pat. No. 3,940,572 issued Feb. 24, 1976, to Robert L. Burgess. This method requires less heat sinking of the transistor than the straight constant current limiting technique above because the short circuit output current is reduced to a fraction of the full load output current. This method however, is undersirable when used with key telephone station instruments containing a microprocessor, because this type of protection circuit, once tripped by an over-current condition, generally requires momentary disconnection of the station to restore station power.
Yet another method of powering a key telephone station instrument while protecting against over-current fault conditions may be realized through use of the circuit described in "Circuits For Engineers," edited by Samuel Weber (page 311). This circuit does not suffer from the limitations cited above in that, upon detecting of an over-current condition the circuit switches its pass transistor completely off thereby eliminating power dissipation through the transistor and thus the need for pass transistor heat sinking. Further, this circuit includes means to delay circuit turnoff on initial detection of an overload condition thereby preventing circuit turnoff by momentary over-current conditions. This design however is undesirable in that, circuit reset may be achieved only by momentarily disconnecting the load or disabling the circuit by means of its input lead. When connected in a multiple protection circuit and load arrangement driven by a common reset signal as is done in the present invention, this circuit reset technique would undesirably cause all protection circuits, whether overloaded or not, to turn off momentarily.
Accordingly it is the object of the present invention to provide a new and improved load protecting arrangement that overcomes the above noted objections.